You put on a used master cylinder? No one is cheaper than me, but even I wouldn't do that. Almost certainly it was damaged when you bled the brakes. By the time a master cylinder gets to be about a year old, crud and corrosion build up in the bottoms of the bores the pistons ride in. You must never, ever push the brake pedal more than half way to the floor or you will run the lip seals over that crud and rip them. At that point you will either get no fluid or the pedal will slowly sink to the floor when you hold steady pressure on it.
Get a professionally-rebuilt master cylinder. You can buy rebuild kits too but they usually cost more than the rebuilt master cylinder. Plus, if you try to rebuild your own, you have to clean the bore completely, and there is a protective anodized coating on it to keep the aluminum from corroding. You can't do anything to scratch or remove that coating.
It IS okay to push the pedal to the floor when bleeding the system with a new master cylinder, but it's good practice to get used to not doing that. Don't forget to bench-bleed it first before you install it. Most rebuilt units come with the instructions and the hoses and fittings to do that.
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Monday, February 17th, 2014 AT 7:02 PM